1. The Field of the Invention
The present invention relates to a power supply circuit capable of actively controlling rise characteristics of an output voltage to be supplied to an electrical load connected to the power supply circuit.
2. Related Art
Power supply circuits, which are required by almost all electronic apparatuses, can be categorized into many types, one of which is a series-regulator type of power supply circuit.
FIG. 1 exemplifies the electronic configuration of such a series-regulator type of power supply circuit 1, which has been used conventionally. This power supply circuit 1 has an input terminal 2 and an output terminal 3, between which a resistor R1 and a transistor Q1 are inserted in series. The transistor Q1 is placed to be controlled by an IC 4. A capacitor C1 is arranged between the input terminal 2 and a ground line 5 for smoothing input voltage, while another capacitor C2 for smoothing output voltage and a resistor R2 (which is a representative of resistive loads) are arranged between the output terminal 3 and the ground line 5.
The IC 4 is in charge of not only constant-voltage control for the transistor Q1 so that a voltage Vo at the output terminal 3 is made to be equal to a target voltage (for example, 5 volts) but also current limiting control to prevent an excessive output current Io. Resistors R3 and R4, which belong to the IC 4 to be connected to the output terminal 3, divide the output voltage Vo to detect a voltage Va. An operational amplifier 6, which is also incorporated in the IC 4, amplifies a difference voltage between the detected voltage Va and a reference voltage Vr indicating a target voltage. The IC 4 also includes transistors Q2 and Q3. One transistor Q2 uses an output voltage from the operational amplifier 6 to drive the transistor Q1. The other transistor Q3, which is electrically connected to a base of the transistor Q2 and the ground line 5, receives control from a current limiter 7 placed in the IC 4. That is, the current limiter 7 drives the transistor Q3 to prevent a voltage across the resistor R1 from exceeding a predetermined limit.
The above power supply circuit 1 is, as one application, applied to an ECU (Electronic Control Unit) mounted to vehicles such as automobiles. In such a case, applying a battery voltage to the input terminal 2 of the power supply circuit 1 will cause the output voltage Vo to rise sharply from a level of zero volts (i.e., causing an overshoot). This overshoot becomes large as a rate of rise of the output voltage Vo becomes fast (i.e., as a rise time becomes shortened). The rise time Tr of the output voltage Vo can be expressed as follows:Tr=C*Vo/Ic  (1),wherein C is a capacitance of capacitive loads (including a capacitor C2) connected to the output terminal 3 and Ic is a charge current flowing into the capacitive loads.
This expression (1) shows that the rise time Tr of the output voltage Vo becomes shorter as the capacitance of the capacitive loads becomes smaller and/or the charge current Ic becomes larger, thus causing an increase in the overshoot.
The above power supply circuit 1 includes the current limiting circuit 7 in order to remove such a problem. Practically, when the current limiting circuit 7 operates to have a smaller current limit, the charge current Ic can be made smaller in amount. However, because it is impossible to lower the current limit than a supply current to the load (resistor R2) during the operation at a rated voltage output, the charge current Ic cannot be set to a lower level if a larger load current is required. Hence, the conventional technique has been obliged to take a countermeasure of, instead of lowering the current limit, giving a larger capacitance to the capacitor C2 such that the overshoot can be suppressed.
This strategy encounters another problem. Specifically, when increasing the capacitance of the capacitor C2 (thus increasing a load capacitance), the capacitor C2 becomes large in the size, leading to an increase in the area of a substrate on which various electrical components are mounted. Therefore, it has been against the demand that a mounting space should be saved and manufacturing cost should be reduced.